I. Gene evolution. Recombinant DNA libraries have been constructed from a variety of wild mouse species. These libraries have been screended with probes corresponding to the Beta chain of the T cell receptor, the immunoglobulin kappa light chain constant region, a kappa variable region gene family and a heavy chain variable region gene family. Sequence analysis of these genes from Mus pahari, the most distantly related mouse species to inbred animals, reveals different patterns of evolution and mutation for each of the genes studied. Thus, selective and evolutionary pressures appear to be acting quite differently on immunologically important structural genes. II. Growth and regulation. A factor produced by the P388D1 macrophage-like cell line has been identified which is required by a number of plasmacytoma cell lines for growth in vitro. This factor is currently being purified and characterized and experiments are being initiated to assess its role in plasmacytomagenesis or normal growth.